Potential Energy Diagram

5 Key excerpts on "Potential Energy Diagram"

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  eBook - ePub

  Survival Guide to General Chemistry

  • Patrick E. McMahon, Rosemary McMahon, Bohdan Khomtchouk(Authors)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  BDE. For calculations, the following equation, applying an enthalpy measurement for potential energy can be used.

  ΔH (reaction) = [total bond BDE (reactants)] − [total bond BDE (products)]

  A conventional chemical reaction describes the process of converting a specific set of reactants to a specific set of products. The corresponding balanced equation, read from left-to-right, describes what is termed the forward reaction.

  Any specific forward chemical reaction can theoretically have an opposite direction reaction. The products of the original forward reaction act as the reactants in the opposite reaction; the reactants of the original forward reaction represent the products of the opposite reaction. Based on the original left-to-right form of the balanced equation, the original reaction is termed the forward reaction direction; the opposite reaction is termed the reverse reaction direction. An important concept (explored further in Chapter 19 ) is the enthalpy relationship between the forward and reverse reactions:

  ΔH (forward reaction) = −ΔH (reverse reaction)

  IV	Potential Energy DiagramS

  Changes in reaction potential energy are very often displayed in the form of a reaction Potential Energy Diagram. This is a pictorial description of a reaction based on plotting the relative potential energies of all reactants and products as a function of the general concept termed reaction progress.

  Reaction progress is non-specific and represents some sequential description of how the reaction changes; reaction progress is shown along the horizontal axis.

  Relative potential energies can be shown as PE or ΔH alone the vertical axis.

  For the simplest form of the diagram, the complete set of initial reactants and the complete set of final products

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  Basic Physical Chemistry

  The Route to Understanding

  • E Brian Smith(Author)
  • 2012(Publication Date)
  • ICP

    (Publisher)

  2 Energy Understanding the energy of chemical systems is of crucial importance to chemists. The properties of atoms and molecules are determined, to a large extent, by the magnitudes of the various forms of energy they contain. The energy changes accompanying chemical processes are a major (but not the only) factor in determining the direction in which reactions can proceed. An understanding of energy was not easily obtained and, although the first tentative steps were made in the 17th century, it was not until the 19th century that the concept was fully established. Nowadays, the most frequently-employed general definitions of energy relate it, somewhat unhelpfully, to the capacity to raise weights. While being aware of such definitions, we will introduce energy in terms of more relevant definitions. 2.1 Kinetic and potential energy The energy possessed by bodies is of two types. The energy which arises by virtue of the motion of a body is referred to as the kinetic energy and is defined by the equation E K = m v 2 2 = p 2 2 m , where m is the mass, v the velocity and p the linear momentum, m v, of the body in motion. In addition to their kinetic energy, bodies can possess potential energy due to the forces that act on them by virtue of their position. The most common example of this is the energy that arises from gravitational forces. This energy, which depends on the height of the body in the Earth’s gravitational field, is termed the gravitational energy, V , and is given by V = mgh , where g is the acceleration due to gravity, which depends somewhat on location but is approximately 9.81 m s − 2 . The kinetic energy of a body at rest is zero. However, the zero of potential energy is arbitrary 17 18 | Basic Physical Chemistry and can be set at a convenient point. Thus, it is a common convention to regard the gravitational potential energy at the surface of the Earth as zero.

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  Engineering Science

  • (Author)
  • 2014(Publication Date)
  • White Word Publications

    (Publisher)

  That is, energy is conserved because the laws of physics do not distinguish between different instants of time. Energy in various contexts The concept of energy and its transformations is useful in explaining and predicting most natural phenomena. The direction of transformations in energy (what kind of energy is transformed to what other kind) is often described by entropy (equal energy spread among all available degrees of freedom) considerations, as in practice all energy transformations are permitted on a small scale, but certain larger transformations are not permitted because it is statistically unlikely that energy or matter will randomly move into more concentrated forms or smaller spaces. The concept of energy is widespread in all sciences. • In the context of chemistry, energy is an attribute of a substance as a consequence of its atomic, molecular or aggregate structure. Since a chemical transformation is accompanied by a change in one or more of these kinds of structure, it is invariably accompanied by an increase or decrease of energy of the substances involved. Some energy is transferred between the surroundings and the reactants of the reaction in the form of heat or light; thus the products of a reaction may have more or less energy than the reactants. A reaction is said to be exergonic if the final state is lower on the energy scale than the initial state; in the case of endergonic reactions the situation is the reverse. Chemical reactions are invariably not possible unless the reactants surmount an energy barrier known as the activation energy. The speed of a chemical reaction (at given temperature T ) is related to the activation energy E , by the Boltzmann's population factor e − E / kT – that is the probability of molecule to have energy greater than or equal to E at the given temperature T . This exponential dependence of a reaction rate on temperature is known as the Arrhenius equation.

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  Fundamental Chemical Kinetics

  An Explanatory Introduction to the Concepts

  • M R Wright(Author)
  • 1999(Publication Date)
  • Woodhead Publishing

    (Publisher)

  This is given here, Figures 6.2a, 6.2b, in terms of the 2-dimensional contour and profile diagrams appropriate to the three atom linear activated complex and the minimum energy path. The features discussed below are common to both the 2-dimensional contour and profile, and also to the P.E. 160 Potential energy surfaces [Ch. 6 corresponding n-dimensional diagrams relevant to the general case where A, B and C can be atoms or molecules. A+BC 'AB 'BC distance along reaction coordinJ Figure 6.2a Potential energy contour diagram showing the reaction coordinate as the minimum energy path, and the position of the critical configuration defining the activated complex. Figure 6.2b The potential energy profile derived from the minimum energy path, the reaction coordinate, in Figure 6.2a At the entrance to the entrance valley r AB is large and decreases towards the col. At the entrance to the exit valley r 8 c is large and decreases towards the col. The position of the col can be described in terms of the relative magnitudes of the distances rAB and rac. The overall energy change on reaction can be classified as exothermic, endothermic or thermoneutral, Figure 6.3. Potential energy surfaces will be discussed in tum for each type of reaction. Exothennic Endothennic P.E. P.E· Thennoneutral l -NJ ---------~-distance along reaction coordinate Figure 6.3 Potential energy profiles for exothermic, endothermic and thermoneutral reactions. ;l/ =O Sec. 6.3) Potential energy contour diagrams 161 6.3.1 Types of potential energy barriers Four types of barrier can be considered, and these are illustrated in the various diagrams ofFigure 6.4.

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  Analytical and Physical Electrochemistry

  • Hubert H. Girault(Author)
  • 2002(Publication Date)
  • EPFL PRESS

    (Publisher)

  1 Electrochemical Potential CHAPTER 1 ELECTROCHEMICAL POTENTIAL 1.1 ELECTROCHEMICAL POTENTIAL OF IONS The chemical potential is the main thermodynamic tool used to treat chemical equilibria. It allows us to predict whether a reaction can happen spontaneously, or to predict the composition of reactants and products at equilibrium. In this book, we shall consider electrochemical reactions that involve charged species, such as electrons and ions. In order to be able to call on the thermochemical methodology, it is convenient to define first of all the notion of electrochemical potential, which will be the essential tool used for characterising the reactions at electrodes as well as the partition equilibria between phases. To do this, let us recall first of all, what a chemical potential is, and in particular the chemical potential of a species in solution. 1.1.1 Chemical potential Thermodynamic definition Let us consider a phase composed of chemical species j. By adding to this phase one mole of a chemical species i whilst keeping the extensive properties of the phase constant, i.e. the properties linked to its dimensions (V, S, n j ), we increase the internal energy U of the phase. In effect, we are adding the kinetic energy E trans , the rotational energy E rot and the vibrational energy E vib if i is a molecule, the interaction energy between the species E int , perhaps the electronic energy E el if we have excited electronic states and the energy linked to the atomic mass of the atoms E mass if we consider radiochemical aspects, such that: U E E E E E E = + + + + + trans rot vib el mass int (1.1) Thus, we define the chemical potential of the species i as being the increase in inter- nal energy due to the addition of this species µ ∂ ∂ i i V S n U n j i =       ≠ , , (1.2) In general, the variation in internal energy can be written in the form of a differential:

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